Two basic direction finding ("DF") techniques have been widely used in the prior art to measure the angular coordinates of incoming electromagnetic radiation. Both techniques employ two or more antennas at the aircraft to receive an electromagnetic signal at the aircraft's location. The signals received by the different antenna elements are compared, whereupon the angular position of a source of the electromagnetic radiation is computed by a processor.
The first of these techniques, designated an amplitude comparison system, compares only the relative amplitude of the signals received by the different antenna elements. This approach is relatively inexpensive to incorporate but also relatively inaccurate. Generally, it can be characterized as including a pair of antennas on an aircraft which are employed to simultaneously but independently receive the incoming signal. The antennas, typically broadband spiral antennas, have apertures squinted off the boresight axis at angles +.theta.s and -.theta.s, respectively. An incoming RF signal approaching at an angle .theta. from the antenna axis is received differently by the antenna elements. The orientation of the antennas A1 and A2 results in the antenna patterns G.sub.1 (.theta.) and G.sub.2 (.theta.). These antenna patterns are typically broad with 3 dB beamwidths generally greater than 60.degree.. As such, a mathematical function can easily be derived which simulates these patterns. The measured amplitude (electromagnetic field strength) of the signals received by the antennas can be compared to one another to determine the angle of arrival .theta. of the source of the signal relative to the axis of the antenna at the aircraft.
While the amplitude comparison system is relatively simple and cost effective, its accuracy is typically on the order of one-tenth of the antenna beamwidth (i.e., about 10.degree.) which is rather poor. Accuracy is basically limited as a result of the inability to accurately measure small differences in amplitude between the received signal at the squinted antenna elements.
A more precise, albeit complex DF approach is known as phase interferometry or phase comparison. According to this technique the pair of antenna elements are separated by a distance "d" and independently receive the transmitted signal. With this approach, the planar apertures of the antennas lie in the same plane rather than being squinted away from one another. To determine azimuth positions, the antennas would be positioned on the y axis; to determine elevation angles, they would lie on the z axis. For the azimuth case, a plane wave propagating toward an aircraft, and arriving at an angle .theta. from boresight (the x axis) is received by each of the two antennas. A phase difference .DELTA..phi. between the signals received by the two antennas is expressed as .DELTA..phi.=2nd sin(.theta.)/.lambda., where .lambda. is the wavelength of the signal propagating from the unknown angular location.
The plane wave travels an extra distance 1=d sin(.theta.) to reach one antenna as compared to the other antenna, thus the phase of the signal received by the first antenna lags accordingly. The phase of the two received signals are compared by a phase comparator and then frequency detected, with the results supplied to a processor where the azimuth angle .theta. of the radiation source is readily computed.
The primary drawback of the phase interferometer approach is that more than one angular position of the target emitter can produce the same phase relationship between the signals received by the two antennas. Consequently, ambiguities in angular position will result with the two antenna approach. The ambiguity problem can be solved by employing one or more additional antennas or pairs of antennas with different baseline spacings between these additional antennas. Ambiguities are then resolved by comparing electrical phase between several pairs of antennas. Once the ambiguities are eliminated, angle of arrival accuracy of the phase interferometry system better than 0.5 degree accuracy has been reported.
However, finding adequate installation locations for the extra antennas renders this type of system impractical, and more so for military aircraft platforms attempting to achieve a small radar cross section.
An attempt to resolve interferometric ambiguities is illustrated in U.S. Pat. No. 5,724,047, entitled "Phase and Time-Difference Precision Direction Finding System", therein phase interferometry is used between planar elements (which are not squinted) to determine the ambiguous angle of arrival. The multiple ambiguities that ensue are resolved by using time-difference-of-arrival (TDOA) of the signal between the two antenna elements. However use of TDOA implies very accurate measurement of time difference in the 10's of picosecond time range, with considerable processing to determine the TDOA by correlation of the signals at the two elements. Accordingly, such an approach results in undesirable more complex and expensive or larger systems.
The accuracy of the angle-of-arrival (AOA) measurements can be degraded if the electromagnetic signal is received from a location that is not at the same elevation as the receiving antennas.
An attempt to reduce elevation induced error is illustrated in U.S. Pat. No. 5,608,411, entitled "Apparatus For Measuring A Spatial Angle To An Emitter Using Squinted Antennas". Therein squinted, doubly polarized (LHCP and RHCP) elements are used to obtain an unambiguous, approximate value of the elevation angle through an induced phase-bias measured by polarization switching at the squinted antennas. However, the use of polarization snitching, phase-bias or other techniques such as is discussed provide only a course estimate of the elevation angle.